It is known that melanin precursors such as 5,6-dihydroxyindoline can be converted into melanin pigments by the oxygen in the air. Based on this knowledge, they have been used for air-oxidative hair dyes.
A melanin precursor used for hair dyes can be prepared by a chemical synthesis reaction, but such a preparation process is accompanied by the problems of, for example, reduction in yield due to side reactions, extra time and costs required for the isolation of a target reaction product, and concern about influence of solvent residue. As a preparation technology of a melanin precursor which makes use of an enzymatic reaction, that of an indole or indoline by using laccase is known (Patent Document 1). The laccase used for this reaction however requires purification from plants such as Japanese lacquer, or the like. In addition, the above-described process is not satisfactory because it is inferior in production efficiency and the melanin precursor thus obtained does not have dyeing properties and is not stable.
In Patent Document 2, on the other hand, melB is disclosed as a novel tyrosinase-encoding gene derived from Aspergillus oryzae. 
[Patent Document 1] JP-A-2002-291496
[Patent Document 2] JP-A-2002-191366